Process for production of negative helium ions and other negative ions

ABSTRACT

A method is disclosed for producing negative ions by passing energetic positive ions through a charge exchange medium consisting of metallic vapors of low ionization potential. This process, in which all low ionization potential metallic vapors can be utilized, enables the production of a large species of negative ions at more copious rates than hitherto possible by charge exchange.

States atent Roy Middleton Moylan;

Charles T. Adams, Wallingford, both of Pa. 815,55 1

Apr. 14, 1969 Nov. 2, 1971 University of Pennsylvania Philadelphia, Pa.

U n ite [72] Inventors [21] Appl. No. [22] Filed [45] Patented [73]Assignee [54] PROCESS FOR PRODUCTION OF NEGATIVE HELIUM IONS AND OTHERNEGATIVE IONS 6 Claims, 2 Drawing Figs.

[52] U.S. Cl 313/63, 250/84, 313/230 [51] Int. CL H05h 5/00 [50]FieldoiSearch 313/63, 230; 250/43, 84

[56] References Cited UNITED STATES PATENTS 3,374,384 3 ll968 ponr ally313/230 PRODUCTION OF NEGATIVE 1/1969 Donnally 313/63 X OTHER REFERENCESNegative Hydrogen lon Source" by .1. A. Weinman and J. R. Cameron, TheReview of Scientific Instruments, Vol. 27, No.5, May 1956, pp. 288- 293,Class 313- 63.

Modification of An lon Source for the Production of Microampere H Beamsby John et al., lEEE Transactions on Nuclear Science, June 1967, VolumeNS-l4, Number 3, pp. 82- 86.

Primary Examiner-Raymond F. Hossfeld Att0mey Paul & Paul 1 ABSTRACT: Amethod is disclosed for producing negative ions by passing energeticpositive ions through a charge exchange medium consisting of metallicvapors of low ionization potential. This process, in which all lowionization potential metallic vapors can be utilized, enables theproduction of a large species of negative ions at more copious ratesthan hitherto possible by charge exchange.

ION SOURCE IONS PATENTEDHUVZ I971 3.617.789

l4 IO 6O Kev ION SOURCE PRODUCTION OF NEGATIVE IONS F l G. l

TANDEM ACCELERATOR FIG. 2.

INVENTORS ROY MIDDLETON 8 CHARLES T. ADAMS WWAM ATTORNEYS PROCESS FORPRODUCTION OF NEGATIVE HELIUM IONS AND OTHER NEGATIVE IONS BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention lies in the fieldof processes for the production of negative ions and, more particularlyto a process for production of negative ions utilizing metallic vaporsof low ionization potential.

2. Description of the Prior Art The production of negative helium ionsis conventionally accomplished by passing positive helium ions through agaseous charge exchange medium, wherein a small fraction of suchpositive ions is converted into negative helium ions. The commonly usedmedium for such charge exchange has been hydrogen. In this standardprocess, the positive ions are produced by a commercially available ionsource and are accelerated to an optimum energy for passage through thecharge exchange medium. The voltage to which the ions are accelerated isdeterminative of the energy of the resulting negative ions.

Relatively recently Donally has disclosed that a greater rate ofproduction of negative helium ions can be produced by injecting lowenergy positive helium ions through a charge exchange medium consistingof the vapor of an alkali atom, Group 1, Periodic Table. See US. Pat.No. 3,374,384, issued Mar. 19, 1968. In particular, Donally utilizedcesium and potassium for the charge exchange medium. Further, heemployed relatively low energies of up to several kilo-electron volts,the optimum energies being said to lie in the range of up to about 3,000electron volts."

Although the process of Donally constituted an improvement over theprior art, it was limited to the vapors of the alkali atoms, and to theproduction of negative helium ions. More particularly, the resultingnegative helium ions carried relatively low energies corresponding tothe optimum energies for producing such negative helium ions. The lowenergy yield has a distinct drawback in that it is generally desirableto have high energy negative ions, as for example, for introduction intoa tandem accelerator.

SUMMARY OF THE INVENTION The primary object of our invention is toprovide a process for producing a large species ofnegative ions.

Another object of our invention is to provide a process for productionof negative ions utilizing metallic vapors of low ionization potential.

It is a further object of our invention to provide a process forproducing negative ions at energies of up to several tens of Kev., andto produce them more copiously than is possible by any prior process,utilizing charge exchange.

Accordingly, this invention provides a process for producing thenegative ions of a plurality of atoms which comprises the introductionof positive ions at relatively high energies into a charge exchangeregion. The medium of such region may be the vapor of any metallic atom,preferably of atoms having low ionization potentials. Upon chargeexchange wherein a fraction of the positive ions introduced into themedium are converted to negative ions, such negative ions are filteredout by conventional electric or magnetic field means. The negative ionsof a plurality of elements, including helium, lithium, boron, carbon,nitrogen, and oxygen, as well as certain radicals, e.g., NH, can beproduced by this process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the process of thisinvention in schematic form.

FIG. 2 shows a schematic representation of a tandem accelerator.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,FIG. I shows a conventional ion source 10 which provides a beam ofpositively charged ions. The positively charged ion beam is acceleratedby a negative voltage V, whereby each positive ion acquires an energy ofV electron volts (assuming that it carries only a sin gle positivecharge). V is termed the charge exchange voltage, said voltagenumerically also representing the energy of the ions in electron-voltsas they pass through the charge exchange region 11. Typical energyvalues as used in this invention lie in the range of 5 to 50 kev. Thecharge exchange region 11 is filled with the vapor of a metallic, lowionization metal, at a pressure optimized for maximum rate of produc'tion of negative ions. Said pressure is a variable, which, along withthe length of the charge exchange region 11, can be adjusted for maximumresults corresponding to the vapor used and the atoms being ionized.Upon emergence from charge exchange region 11, the negative ions will befurther accelerated due to the potential field effects of groundelectrode 14, whereby the energy of each negative helium atom will bedoubled. Consequently, if V is made 30 kv., the negative atoms willemerge at the end of the process with 60 kev. energy. The negative ionswhich have thus been produced can be conveniently separated from theremainder of the positive ions by conventional electric or magneticfield techniques.

Experiments performed utilizing the process of this invention havedetermined that high energy beams of negative heliurn ions can beproduced by charge exchange in a wide variety of vapors in addition tothose of the alkali metals, Group I, Periodic Table. In particular, thevapors of the alkaline earth metals, Group II, Periodic Table, are veryeffective. The vapor properties which appear to be most determinative ofthe efficiency of the production of beams of negative helium and otherions are that the vapor be metallic and have a low ionization potential.In general, the lower the ionization potential the greater is the yieldof negative ions. In addition, it is desirable that the vapor should beconstituted of atoms having a low atomic number, in order to minimizethe multiple scattering of the ion beam which results from collisionswith the vapor atoms. In this respect, for example, magnesium ispreferable to barium.

While the process has been described in terms of vapors comprised ofmetallic atoms, beams of negative ions can be produced at desirableenergies by charge exchange in the vapors of other low ionizationpotential elements, as well as compound vapors. It is anticipated thatany compound having a low ionization potential, or containing atomshaving a low ionization potential which would be released uponcollision, would be a suitable vapor medium.

The use of high voltages, and correspondingly high charge exchangeenergies, plays a dual role in the process of this invention. First, thehigher energies are required to effect charge exchange. In using thevapor of lithium, a Group 1 atom, for the charge exchange medium,intense beams of negative He ions were produced at an exchange energy ofapproximately 20 kev., and a slightly higher yield of He at an exchangeenergy of approximately 26 kev. Similarly, positive lithium ions withexchange energies in the range of 30 to 40 kev., when passed throughlithium vapor, produced a yield of negative lithium ions about an orderof magnitude more efficient than that possible with the standardhydrogen process.

The second beneficial role played by using high exchange energies isthat the resulting negative ions are produced at higher energies. Thisis of critical importance in the operation of a tandem accelerator,where the negative ions must be introduced at high energy in order toavoid beam scattering, preferably approaching I00 kev. In a typicalapplication of this invention, the negative ion which has been produced,carrying, for example, 60 kev., as noted in FIG. 1, is introduceddirectly into the tandem accelerator shown in FIG. 2. If such negativeion carries a low .energy of only several kev., it becomes necessary topass it through a separate preacceleration stage prior to introductionto the tandem accelerator. The subject process yields negative ionshaving twice the charge exchange energy, typically 50 to 80 kev., arange which is acceptable for tandem accelerator operation.

The process of this invention is general in that it is not restricted tothe production of negative helium ions. Rather, a wide range of negativeions can be produced by charge exchange in a vapor medium possessing alow ionization potential. It was stated above that positive lithium ionsof energy in the range of 30 to 40 kev., when passed through lithiumvapor, yield a beam of negative ions approximately one order ofmagnitude greater than that produced by using hydrogen as the chargeexchange medium. Similarly, positive boron ions and carbon ions, whenpassed through lithium vapor have yielded large negative ion currents.Our investigations also have shown that such other negative ions can beproduced copiously by charge exchange in other than vapors of thealkaline metals, Group 1, Periodic Table. Indeed, the vapors of thealkaline earth metals, Group I], Periodic Table, and other lowionization potential metallic elements and compounds, are almost aseffective. All such negative ions, produced by the process of thisinvention, possess the additional advantage of carrying higher energies.

The following table summarizes the results obtained using the process ofthis invention. Current figures indicate relative negative ion yieldswith respect to a reference source of positive ion current.

Charge Ion exchange current, Vapor Ionization energy, micro- Ion elementGroup Z potential kev. amps II 12 7. 61 26 7 II 20 6. 09 10-35 6-7 I 35. 36 20 9 I 3 5. 36 26 9 VI 16 10. 30 20 05 25 09 30 12 35 15 40 16 4522 r 50 24 3110 Zn 11b 30 9 36 25 1. 2 1. 6 1. 7 1. 85 1. 95 2. 0 Li- LiI 3 5. 36 2040 3-5 Li I 3 5. 36 20-40 3-4 Li 1 3 5. 36 3040 5-10 N i 1 35. 36 40 O- or OII- Li I 3 5. 36 40 100 Transition element. 7 V

The comparison between Zinc and sulfur confirms that the yield isoptimized for a metallic exchange medium having a low ionizationpotential. The vapors of Group 11 elements are appreciably as effectiveas Group l-elements, the effectiveness dropping off with increasingionization potential.

in general, any low ionization potential metal which can be vaporizedcan be used in this process, for the production of any negative ion.Although charge exchange energies in the range of 10 to 50 kev. havebeen used in experiments to date, higher energies should provide greateryields of certain negative ions, particularly heavier ions, wheninteracting with certain metallic vapors. Further, charge exchangeenergies in the range of 5 to 10 kev. should be effective in producingcertain negative ions, particularly lighter ions. Although the processof this invention has been described with reference to specific ions,vapors and energies, modifications can be made within the spirit andscope of this invention.

We claim:

1. A process for producing negative helium ions, comprisa. acceleratinga beam of positive helium ions through a negative potential in the rangeof5 to 50 kv., thereby raising the energies ofsaid ions to 5 to 50 kev.;and

b. passing said beam of positive helium ions through a vapor, said vaporcomprising atoms selected from the group consisting of the alkalineearth metals, Group 11, Periodic Table, said energized positive heliumions interacting with said vapor whereby some of said positive heliumions undergo charge exchange and are converted to negative helium ions.

2. The process as defined in claim 1 wherein said negative helium ionsare attracted to ground potential, thereby raising the energies of saidnegative helium ions to the range of 10 to kev.

3. A process for producing negative helium ions comprising:

a. accelerating a beam of positive helium ions through a negativepotential in the range of5 to 50 kv., thereby raising the energies ofsaid positive ions to the range of 5 to 50 kev.;

b. passing said beam of energized positive ions through a vapor, saidvapor comprising atoms selected from the group consisting of magnesium,calcium, sulfur and zinc, said energized positive helium ionsinteracting with said vapor such that some of said positive helium ionsundergo charge exchange and are converted to negative helium ions.

4. A process for producing negative ions selected from the groupconsisting of Li, B, C, O, NH, and OH, comprising:

a. accelerating a beam of positive ions through a negative potential inthe range of 5 to 50 kv., said positive ions being selected from thegroup consisting of Li, 8*, C*, 0*, NH", and OH*; and

b. passing said beam of positive ions through a vapor, said vaporcomprising atoms selected from the group consisting of the alkalimetals, Group I, Periodic Table, said accelerated positive ionsinteracting with said vapor whereby some of said positive ions undergocharge exchange and are converted to negative ions.

5. The process as described in claim 4 wherein said vapor comprisesatoms of lithium.

6. A process for producing negative ions selected from the groupconsisting of Li, B, C, O, NH and OH, comprising:

a. accelerating a beam of positive ions through a negative potential inthe range of 5 to 50 kv., said positive ions being selected from thegroup consisting of Li", 8*, C", 0*, NH*, and OH*; and

b. passing said beam of positive ions through a vapor, said vaporcomprising atoms selected from the group consisting of the alkalineearth metals, Group ll, Periodic Table, said energized positive ionsinteracting with said vapor whereby some of said positive ions undergocharge exchange and are converted to negative ions.

:1: a a a:

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,617Dated November 2,. '1971 lnventofls) Y leton et a1 It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 68, "NH should read NH Column 2,

line 57, 3Hel" should read He' line 58, He should read Ee---. Column 4,line 38, "Li B 0 NH and CH should read Li, B, C, O, NH, and OH line 52,

Li B C 0 NH and 0H should read Li, B, C, O,

NH, and OH" Signed and sealed this 9th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents JRM PO-IOEO USCOMM-DC coa'Io-paa Q U 5, GOVERNMENY PRINTINGOFFICE I"! (3-365-11

2. The process as defined in claim 1 wherein said negative helium ionsare attracted to ground potential, thereby raising the energies of saidnegative helium ions to the range of 10 to 100 kev.
 3. A process forproducing negative helium ions comprising: a. accelerating a beam ofpositive helium ions through a negative potential in the range of 5 to50 kv., thereby raising the energies of said positive ions to the rangeof 5 to 50 kev.; b. passing said beam of energized positive ions througha vapor, said vapor comprising atoms selected from the group consistingof magnesium, calcium, sulfur and zinc, said energized positive heliumions interacting with said vapor such that some of said positive heliumions undergo charge exchange and are converted to negative helium ions.4. A process for producing negative ions selected from the groupconsisting of Li , B , C , O , NH , and OH , comprising: a. acceleratinga beam of positive ions through a negative potential in the range of 5to 50 kv., said positive ions being selected from the group consistingof Li , B , C , O , NH , and OH ; and b. passing said beam of positiveions through a vapor, said vapor comprising atoms selected from thegroup consisting of the alkali metals, Group I, Periodic Table, saidaccelerated positive ions interacting with said vapor whereby some ofsaid positive ions undergo charge exchange and are converted to negativeions.
 5. The process as described in claim 4 wherein said vaporcomprises atoms of lithium.
 6. A process for producing negative ionsselected from the group consisting of Li , B , C , O , NH and OH ,comprising: a. accelerating a beam of positive ions through a negativepotential in the range of 5 to 50 kv., said positive ions being selectedfrom the group consisting of Li , B , C , O , NH , and OH ; and b.passing said beam of positive ions through a vapor, said vaporcomprising atoms selected from the group consisting of the alkalineearth metals, Group II, Periodic Table, said energized positive ionsinteracting with said vapor whereby some of said positive ions undergocharge exchange and are converted to negative ions.